Ferritic alloy with high temperature strength containing dispersed intermetallic TiSi

ABSTRACT

An iron-aluminum ferritic alloy characterized by hightemperature strength, resulting from the precipitation of a fine TiSi intermetallic type precipitate of high thermal stability, and high-temperature oxidation-resistance under conditions of cyclic heating at temperatures up to about 2,500* F. said alloy consists essentially of, by weight, up to about 0.1% carbon, from about 4.0 to about 8.2% aluminum, about 0.2 to 4.0% silicon, about 0.05 to 2.0% titanium, with a preferred silicon/titanium ratio between about 1.0 to 4.0, and the balance iron, incidental impurities and additions which do not materially affect the attainment of the desired properties.

llnited States Patent [191 Giles et al.

[ Mar. 25, 1975 FERRlTIC ALLOY WITH HIGH TEMPERATURE STRENGTH CONTAINING DISPERSED INTERMETALLIC T181 [75] Inventors: Philip M. Giles, Bethlehem; I-Ialle Abrams, Allentown; Arnold R. Marder, Bethlehem, all of Pa.

[73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

[22] Filed: July 20, 1973 [21] App]. No.: 381,288

[56] References Cited UNITED STATES PATENTS 5/1972 Matsuho 75/124 7/1972 Cooper 75/124 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmJoseph J. OKeefe; William B. Noll [57] ABSTRACT An iron-aluminum ferritic alloy characterized by hightemperature strength, resulting from the precipitation of a fine TiSi intermetallic type precipitate of high thermal stability, and high-temperature oxidationresistance under conditions of cyclic heating at temperatures up to about 2,500 F. said alloy consists essentially of, by weight, up to about 0.1% carbon, from about 4.0 to about 8.2% aluminum, about 0.2 to 4.0% silicon, about 0.05 to 2.0% titanium, with a preferred silicon/titanium ratio between about 1.0 to 4.0, and the balance iron, incidental impurities and additions which do not materially affect the attainment of the desired properties.

10 Claims, 1 Drawing Figure TEN/ EM Tums, "F x 0 ll FERRITIC ALLOY WITH HIGH TEMPERATURE STRENGTH CONTAINING DISPERSED INTERMETALLIC TISI CROSS-REFERENCES TO RELATED APPLICATION BACKGROUND OF THE INVENTION This invention relates to a high-strength, low-cost iron-aluminum ferritic alloy, characterized by a microstructure containing a fine TiSi intermetallic type precipitate dispersed uniformly throughout the matrix of the grains thereof. The alloy also possesses hightemperature oxidation-resistance, a characteristic ideally suited for application where thermal shock, such as cyclic heating and cooling, are experienced. While not intending to unduly limit this invention, applications which can take advantage of the properties hereof are high temperature exhaust systems in automobiles, jet engines, and in the petrochemical industry.

Heretofore, high-temperature oxidation-resistant materials were selected from high cost nickel and cobalt superalloys, austenitic stainless steels, or ceramic materials. In an effort to lower cost, the prior art moved to lower or less rich alloys. For example U.S. Pat. No. 1,641,752 teaches a ferrous alloy resistant to oxidation at high temperatures, by including therein a high percentage of aluminum. Specifically, said alloy contains from 12 to 20% aluminum, and about 1 to 5% ofa grain refining material, among which are included titanium and chromium. The high-temperature oxidationresistance is due at least in part to the formation of a protective coating of oxide of the aluminum on the exposed surfaces of the ferrous alloy. However, such alloys are of limited suitability under cyclic heating and cooling conditions wherein thermal shock results in flaking and spalling of the oxide coating. Accordingly, one of the critical requirements ofa suitable alloy is its ability to resist such flaking and spalling.

A more recent development covering hightemperature oxidation-resistant alloys is described in U.S. Pat. No. 3,192,073. This patent teaches a method for making an iron-alumium alloy product fortified for high-temperature environments by the application to the said product of an aluminum coating, followed by a high temperature anneal to diffuse said coating into the alloy product. The oxidation-resistance of the coated and heat treated product is enhanced by the fur- .ther development of iron-aluminum compounds near the surface thereof. In other words, a protective layer or enriched case is formed about the said product. Over and above the costly treatments for coating and diffusion annealing, there is a serious question of whether such a product could withstand the cyclic heating and cooling which is incidental to many industrial applications. For example, the material of the latter patent is characterized by an aluminum gradient between the coating and core, and as a result would have different densities and coefficients of expansion. Thermal cracks could develop from said cyclic treatments, as well as an eventual deterioration thereof due to continued diffusion and depletion of the aluminum from the coating.

As is evident from the preceding discussion, ironaluminum alloys have served as a base alloy for developing high-temperature oxidation-resistant alloys. U.S. Pat. No. 3,698,964, represents a further effort in this direction. Briefly, the alloy of said patent is an iron base alloy containing chromium and aluminum and/or silicon. More specifically, the alloying additions thereto are on the order of from 1 to 5%. The improved oxidation-resistance, or decrease in weight gain with temperature, is attributed to the formation of alumina, or a complex oxide of iron and aluminum. While such an alloy appears to exhibit superior oxidation-resistance when compared to a Type 301 austenitic stainless steel, no mention is made as to its high temperature strength. In contrast to this, the present invention teaches a ferritic alloy which is not only oxidation-resistant at elevated temperatures, but possesses high strength in conjunction therewith.

SUMMARY OF THE INVENTION The present invention relates to a high-strength ironaluminum ferritic alloy which is not only resistant to oxidation at high temperatures, but is resistant to surface flaking and spalling when subjected to thermal shocking as a result of cyclic heating and cooling. More particularly, said invention covers an alloy consisting essentially of, by weight, up to about 0.1% carbon, about 4.0 to 8.2% aluminum, up to about 10.5% chromium, about 0.2 to 4.0% silicon, about 0.05 to 2.0% titanium, with a preferred silicon titanium ratio between about 1.0 to 4.0 and the balance iron, incidental impurities and additions which do not materially affect the attainment of the desired properties. The unexpected high-temperature strength is derived from the precipitation at very high temperatures, ofa fine TiSi intermetallic type precipitate of high thermal stability, said precipitate being uniformly dispersed throughout the matrix of the grains thereof.

BRIEF DESCRIPTION OF DRAWING The FIGURE is a graph showing the increase or change in strength, over a range of elevated temperatures, for two alloys of this invention (base alloys plus titanium and silicon according to the teachings herein) and base alloys plus no more than one of the elements from the group consisting of silicon, titanium, or an increase in the carbon content.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT This invention is directed to the family of iron-aluminum alloys, more particularly to the wrought ferritic alloysthereof which may be employed in appli cations where the ability to withstand thermal shock is critical. The alloys hereof are characterized by a combination of high-strength and oxidation-resistance under conditions of cyclic heating at temperatures up to about 1,800 F. Typical applications where the latter conditions prevail are high temperature exhaust systems, such as a thermal reactor in automobiles, jet engines, and in the petrochemical industry. While the mention of same is not intended as a limitation on this invention, it is believed that a brief discussion thereof will help in understanding the significance of this invention and the contributions offered thereby.

Generally, a thermal reactor is a container into which the hot exhaust gases flow from the automotive, jet or power producing engines, for further combustion. Air is also pumped into the reactor and admixed with said gases. The reactor is generally of sufficient size (chamber volume) to give a long enough residence time of the believed that the prior art establishes the general performance of the iron-aluminum alloys to resist hightemperature oxidation, while said copending application establishes the specific capabilities of the alloys admixed gases to permit complete combustion of the 5 hereof to perform at temperatures up to about 2,500 residual hydrocarbons and carbon monoxide. Since the F. Accordingly, the data therefrom is incorporated by combustion reaction is so strongly exothermic, temperreference and will not be presented here. atures may go as high as 2,200 F., typically about From this then, the significantfeature of the present l,800 F., in a highly oxidizing environment. Thus, the invention becomes the unexpected increase in strength severe conditions of high temperature, oxidizing enviof the alloys as a result of the precipitation ofa fine TiSi ronment, and intermittent operation (cyclic heating intermetallic type precipitate of high thermal stability. and cooling), call for a material capable of meeting The synergism resulting from the controlled addition of th se conditi ns, The alloys of th present invention both titanium and silicon is believed clearly demonnot only fulfill these conditions, but accomplish it at a S ed from he FIGURE, and TABLE ll below. For low material cost. That is, as a result of the unexpected this mo ration, t O base alloys. Alloys A and C of increase in strength less material is required, Table l, were tested 218 to their strength at varying ele- The composition of the alloys of this invention fall vated temperatures. To several other alloys having broadly within the alloying ranges below: chemistries comparable to the base alloys were added Ca b up t b t 01 t the elements titanium, carbon and/or silicon. The alloy Al i 40 to 82 wt, designations with their melt chemistries, by weight per- Chromium up t ab t 10,5 wt, cent, are listed below in TABLE 1. Silicon 0.2 to 4.0 wt. i

lron balance, with a preferred silicon/titanium ratio between about Alloy Carbon Chm Alumi Titanium Silicon 1.0 to 4.0. Within said broad ranges, a preferred commium num position is one containing at least about 8.0% chro- 7 mium, aluminum between about 6.0 to 7.0%, titanium '3 '8}? 3'8 2'; :8; between about 0.2 to 1.2%, silicon between about 0.6 c I012 79 I004 ,01 to 1.8%, and a titanium/carbon ratio of at least 4.0. B 8g: g8 -25 It will be recalled from the data presented in said co- F I I pending application that there is a general relationship G 023 between the performance of the alloys of the type herein under high-temperature conditions to the rich- To secure the date of TABLE II, the various alloys ness or total alloying content of the alloy. While this 35 listed above were vacuum-induction melted and vacugeneral proposition may suggest improved performum-cast into ingot molds resulting in ingots weighing ance with the richer alloys, there are some practical about 400 lbs. After slow cooling to ambient temperalimits due to alloying costs and processing restrictions. tures, the ingots were slowly heated (at least 5 hours) in this regard, the production of wrought strip and to a soaking temperature between about 2,025 to sheet by conventional rolling practices requires that the 2,130 F. For the initial hot rolling, the ingots were rematerial be sufficiently ductile for said rolling to be efduced at about /2 inches per pass to a plate thickness fected. In the case of aluminum containing iron base of about 1 inch. The finishing temperatures varied bealloys, the ductility thereof drops below a practical tween about 1,650 to 1,850 E, and from here the level when the aluminum begins to exceed about 8% by plates were slowly cooled to ambient temperatures by weight. Chromium, when added to an Al-Fe alloy, burying in sand. Tensile test specimens were then mashould not exceed about 10% by weight, as the risk of chined from the plates and tensile tests were then perbreakage and other processing problems increases. Fiformed on the material. Results of the tensile tests are nally, with respect to carbon it should be l eptto a Low reported inTable ll. TABLE [I TENSILE STRENGTH (KSl) at at at at at Alloy 1 150 F. 1200 F. 1400 F. 1000 F. 1800 F.

A 29.5 12.0 4.5 2.5 B 41.5 21.5 10.0 4.5 C 29.0 v.0 7.0 4.4 D 37.9 V3.8 8.2 4.3 E 37.4 17.6 7.3 4.7 F 36.9 17.0 8.0 3.2 G 42.6 20.7 9.2 4.9

value, preferably less than about 0.04%, typically less While TABLE II presents the results in a form as acthan 0.03%. But in no case should it exceed 25% of the tually measured, a different approach was taken with titanium. the FIGURE. For the graphic demonstration, the

From the outset of this description, emphasis has been placed on the two principal characteristics of the alloys of this invention; namely, high-temperature oxidation-resistance, and high-temperature strength. It i s strength difference between an appropriate base or reference alloy (the particular base alloy is designated on each curve) and the base alloy plus additives was plotted along the ordinate. The base alloy used for Alloy E was Alloy F as the carbon difference was less than if Alloy C had been used.

The clear beneficial relationship of the combination of titanium and silicon can best be seen from a comparison of Alloys D and E with Alloy G. From TABLE I it will be observed that to the base chemistry of Alloys D and E, only one of the elements titanium and silicon were included. If, for example, the strength increase attributed to each such element individually were added to each other, the result would still fall far short of that actually realized with Alloy G.

This unexpected improvement may best be illustrated by the FIGURE. The curve (0,; Up) represents the difference in strength observed at various temperatures, when the titanium containing Alloy E is contrasted to Alloy F, comparable in all regards except the titanium content. In similar fashion, curve (0,, 0 represents the difference in strength at the varying temperatures between Alloy D and Alloy C. The alloys were comparable except for the 0.61% by weight silicon in the former. By these comparisons it was possible to essentially isolate the effect of the individual element, titanium or silicon, on the high temperature strength of the alloys. By adding together the strength changes at a given temperature, and comparing it to the curve (0' a it will be seen that the result is not merely additive, but significantly higher than expected. This synergism is attributed to the development of a TiSi intermetallic type precipitate which has been observed in the matrix of the grains thereof.

Transmission electron microscopic examination of the silicon-titanium bearing alloys of this invention revealed a structure containing a very small (100 to 600A), well dispersed, spherical precipitate of very high thermal stability throughout the matrix of the said grains. Although a positive identification of the precipitate could not be made from currently available ASTM file data, the correspondence between the observed pattern and that of the face-centered-cubic Fe Si (a 5.64A.) is quite good. If it is postulated that some of the iron atoms are replaced by the larger titanium atoms (the radii of the atoms are 1.26 A. for iron versus 1.49 A. for titanium) it becomes very reasonable to believe that the observed precipitate is of the form (Fe, Ti) Si with an a, 5.80A. and with a face-centered-cubic structure.

Thus, by the discovery of the unique strengthening mechanism attributed to the development of said TiSi intermetallic type precipitate, a high temperature strength version of the iron-aluminum ferritic alloys evolved.

We claim:

1. A high temperature high strength ferritic alloy characterized by a microstructure containing a small, well dispersed, spherical TiSi type intermetallic precipitate and consisting essentially of, by weight, up to about 0.1% carbon, from about 4.0 to about 8.2% aluminum, up to about 10.5% chromium, from about 0.2 to about 4.0% silicon, from about 0.05 to about 2.0% titanium, where the silicon content is at least equal to the titanium content to insure the development of said precipitate, with the balance iron.

2. The ferritic alloy claimed in claim 1 wherein the silicon/titanium ratio is between 1.0 to about 4.0.

3. The ferritic alloy claimed in claim 1 wherein the carbon content is less than about 0.04%.

4. The ferritic alloy claimed in claim 3 wherein the maximum carbon content is 0.03%.

5. A ferritic alloy characterized by high temperature strength and consisting essentially of, by weight, up to about 0.1% carbon, from about 6.0 to about 7.0% aluminum, from about 8.0 to about 10.5% chromium, from about 0.6 to about 1.8% silicon, from about 0.2 to about 1.2% titanium, where the silicon content is at least equal to the titanium content, with the balance iron.

6. The ferritic alloy claimed in claim 5 wherein the silicon/titanium ratio is between 1.0 to about 4.0.

7. The ferritic alloy claimed in claim 5 wherein the carbon content is less than about 0.04%.

8. The ferritic alloy claimed in claim 7 wherein the maximum carbon content is 0.03%.

9. The ferritic alloy claimed in claim 5 wherein the microstructure is characterized by a structure containing a small, well dispersed, spherical intermetallic precipitate.

10. The ferritic alloy claimed in claim 5 wherein said precipitate is a TiSi type intermetallic precipitate. 

1. A HIGH TEMPERATURE HIGH STRENGTH FERRITIC ALLOY CHARACTERIZED BY A MICROSTRUCTURE CONTAINING A SMALL, WELL DISPERSED, SPHERICAL TISI TYPE INTERMETALLIC PRECIPITATE AND CONSISTING ESSENTIALLY OF, BY WEIGHT, UP TO ABOUT 0.1% CARBON, FROM ABOUT 4.0 TO ABOUT 8.2% ALUMINUM, YP TO ABOUT 10.5% CHROMIUM, FROM ABOUT 0.2 TOA BOUT 4.0% SILICON, FROM ABOUT 0.05 TO ABOUT 2.0% TITANIUM, WHERE THE SILICON CONTENT IS AT LEAST EQUAL TO THE TITANIUM CONTENT TO INSURE THE DEVELOPMENT OF SAID PRECIPITATE, WITH THE BALANCE IRON.
 2. The ferritic alloy claimed in claim 1 wherein the silicon/titanium ratio is between 1.0 to about 4.0.
 3. The ferritic alloy claimed in claim 1 wherein the carbon content is less than about 0.04%.
 4. The ferritic alloy claimed in claim 3 wherein the maximum carbon content is 0.03%.
 5. A ferritic alloy characterized by high temperature strength and consisting essentially of, by weight, up to about 0.1% carbon, from about 6.0 to about 7.0% aluminum, from about 8.0 to about 10.5% chromium, from about 0.6 to about 1.8% silicon, from about 0.2 to about 1.2% titanium, where the silicon content is at least equal to the titanium content, with the balance iron.
 6. The ferritic alloy claimed in claim 5 wherein the silicon/titanium ratio is between 1.0 to about 4.0.
 7. The ferritic alloy claimed in claim 5 wherein the carbon content is less than about 0.04%.
 8. The ferritic alloy claimed in claim 7 wherein the maximum carbon content is 0.03%.
 9. The ferritic alloy claimed in claim 5 wherein the microstructure is characterized by a structure containing a small, well dispersed, spherical intermetallic precipitate.
 10. The ferritic alloy claimed in claim 5 wherein said precipitate is a TiSi type intermetallic precipitate. 